The optimum functioning of a furnace can be considered from two points of view, namely from the point of view of fuel consumption and from the point of view of emission of noxious gases.
In order to meet legal requirements regarding the maximum allowable emission of noxious gases or pollutants and, at the same time, to maximize the use of fuel, furnaces must be operated with excess air of between 5% and 40% (air number .lambda.=1.05-1.4), the optimum value depending upon the type of furnace and boiler used.
However, the air excess should not exceed that which is absolutely essential, since increases in excess air, while decreasing the emission of pollutants, at the same time decrease the efficiency of the furnace since a greater amount of ballast air must be warmed and discharged through the chimney.
The partial pressure of oxygen in the exhaust gases is determined by the amount of excess air during combustion. The higher the excess air, that is the higher the amount of air relative to that required for complete combustion, the larger the partial pressure of oxygen in the exhaust gases.
The amount of excess air during combustion can thus be monitored for purposes of control and regulation of the operating conditions of the furnace by measuring the partial pressure of oxygen in the exhaust gases.
This principle has been used for several applications, sensors having been developed in particular for monitoring the emission of pollutants of internal combustion engines by measuring the partial oxygen pressure in the exhaust gases of automobiles.
In automobiles, a closed loop control circuit is provided which utilizes the sensor output to stablize the air-fuel mixture to the value .lambda.=1, since, for a stoichiometric air/fuel ratio an almost complete elimination of pollutants can be accomplished by a subsequent catalytic converter. For a gas mixture in thermodynamic equilibrium, the partial oxygen pressure in the exhaust gas at .lambda.=1 changes abruptly by several factors of ten. The associated jump in the signal furnished by the sensor allows a relatively simple electronic regulation of the air/fuel ratio to the value .lambda.=1.
Since, for furnaces, no catalytic after-reaction of the exhaust gases takes place, the combustion should be carried out with an air number .lambda.&gt;1 to minimize the emission of pollutants.
These relationships are published in a number of publications. In the Motortechnischen Zeitschrift 34 (1973) 1, S. 7, R. Zechnall and G. Baumann give a general overview of the purifying of exhaust gases of Otto motors. Closed loop control circuits are used. The sensors are so-called .lambda. sensors which generate a voltage which varies as a function of the partial pressure of oxygen in the exhaust gas. The basic operating principle of such a sensor includes the comparision of the partial oxygen pressure in the exhaust gas and that in the surrounding air by means of a solid electrolyte (zirconium dioxide) which is conductive to oxygen ions and which separates the two gases from each other. Electrodes are affixed to the electrolyte and a so-called Nernst voltage is generated between the electrodes which varies as a function of the partial pressure of oxygen on each side. It also depends upon the temperature at which the sensor is maintained. The output voltage of such a zirconium dioxide sensor undergoes a rapid change at the value .lambda.=1, while the variation as a function of partial oxygen pressure for .lambda.&lt;1 and .lambda.&gt;1 is very small.
A further principle for monitoring the partial oxygen pressure in exhaust gases of automobiles is described by Tien, Stadler, Gibbons and Zacmanidis in Ceramic Bulletin Vol. 54, No. 3 (1975) page 280. Here the quasicontinuous oxygen reduction of TiO.sub.2 in dependence of the partial oxygen pressure in the surrounding gaseous atmosphere is used for measuring the partial pressure of oxygen.
Discharge of oxygen causes the electrical conductivity of titanic oxide to change continuously. The variation of resistance of such a sensor as a function of air number .lambda. has a similar variation as the voltage of an emf sensor. In particular, the variation of resistance as a function of partial oxygen pressure is very small for .lambda.&gt;1.
A similar sensor is described in U.S. Pat. No. 1,467,735.
In known sensors for partial oxygen pressure in flue gas and furnace installations similar sensors to those described in relation to automobiles are used. In the system described in German published application No. DE-AS 2,400,246, a solid electrolyte cell of zirconium oxide is arranged in the stream of the exhaust gas and air is used as a comparison gas. In order that the partial oxygen pressure may be measured by such a sensor, it must be temperature-stablized. For this purpose in the above-mentioned German patent, a heating coil is wound around the sensor. The evaluation and further processing of the output signal of such a sensor is very difficult because of its above-mentioned very small variation as a function of partial oxygen pressure. Published German application No. 2,510,189 describes the use of a zirconium oxide sensor in the flue gas of a furnace for direct control of the burner, that is for regulation of the air/fuel ratio. Such installations have been used in practice, but their application is limited to very large furnaces, since the costs of a control system utilizing zirconium oxide sensors is very high. These high costs are the result of the small variation of output signal of the sensor as a function of partial oxygen pressure, since this leads to very complicated and expensive electronic circuits if the fuel/air ratio is to be stablized within a narrow .lambda. region for .lambda. values &gt;1.
To overcome these difficulties, attempts have been made to increase the variation of the output signal of the sensor as a function of partial oxygen pressure by adding an auxiliary gas (hydrogen) to the exhaust gas to be monitored. In thermodynamic equilibrium, the addition of the auxiliary gas causes the signal change which otherwise takes place at .lambda.=1 to occur at values of .lambda.&gt;1. The value of .lambda. at which the sudden jump occurs depends upon the ratio of exhaust gas flow to auxiliary gas flow. For this arrangement, the problem of maintaining the ratio of exhaust gas flow to auxiliary gas flow constant requires approximately as much equipment and therefore is of approximately the same cost as the problem of maintaining the air/fuel ratio at the input of the burner constant.
To allow the application of monitoring and control systems for exhaust gas in household furnaces, inexpensive and simple measuring systems must be developed which do not require an auxiliary gas as a reference and whose output signal in the .lambda. region of interest has a strong variation as a function of partial oxygen pressure.